Re: [dpdk-dev] [EXT] Re: [PATCH v2 2/2] eal: roundup tsc frequency when estimating it

[Pavan Nikhilesh Bhagavatula <pbhagavatula@marvell.com>]

On Sat, 2019-03-16 at 18:22 +0000, Wiles, Keith wrote:
> > On Mar 16, 2019, at 12:56 PM, Pavan Nikhilesh Bhagavatula <
> > pbhagavatula@marvell.com> wrote:
> > 
> > On Sat, 2019-03-16 at 17:18 +0000, Wiles, Keith wrote:
> > > > On Mar 16, 2019, at 10:06 AM, Pavan Nikhilesh Bhagavatula <
> > > > pbhagavatula@marvell.com> wrote:
> > > > 
> > > > On Sat, 2019-03-16 at 14:42 +0000, Wiles, Keith wrote:
> > > > > > On Mar 16, 2019, at 2:03 AM, Pavan Nikhilesh Bhagavatula <
> > > > > > pbhagavatula@marvell.com> wrote:
> > > > > > 
> > > > > > From: Pavan Nikhilesh <pbhagavatula@marvell.com>
> > > > > > 
> > > > > > When estimating tsc frequency using sleep/gettime round it
> > > > > > up
> > > > > > to
> > > > > > the
> > > > > > nearest multiple of 10Mhz for more accuracy.
> > > 
> > > How does rounding up the TSC value become more accurate, If the
> > > value
> > > is 1 cycles more then it should be then your macro would round
> > > down
> > > and if it is 1 cycle greater than 1E7 it would round up.
> > 
> > Example in case of RTE_ARM_EAL_RDTSC_USE_PMU enabled 
> > 
> > Before roundup : 1400000979
> > After roundup : 1400000000
> > EAL: TSC frequency is ~1400000000 Hz
> > 
> > 
> > Before roundup : 1399999060
> > After roundup : 1400000000
> > EAL: TSC frequency is ~1400000000 Hz
> > 
> > > > > > Signed-off-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
> > > > > > ---
> > > > > > Useful in case of ARM64 if we enable
> > > > > > RTE_ARM_EAL_RDTSC_USE_PMU,
> > > > > > get_tsc_freq_arch() will return 0 as there is no
> > > > > > instruction to
> > > > > > determine
> > > > > > the clk of PMU and eal falls back to sleep(1).
> > > > > > 
> > > > > > lib/librte_eal/common/eal_common_timer.c | 4 ++--
> > > > > > lib/librte_eal/linuxapp/eal/eal_timer.c  | 2 +-
> > > > > > 2 files changed, 3 insertions(+), 3 deletions(-)
> > > 
> > > It appears you did not use the head of the master as linuxapp is
> > > now
> > > just linux and freebsdapp is freebsd. You need to rebase to the
> > > head
> > > of master and send a new version.
> > > > > > diff --git a/lib/librte_eal/common/eal_common_timer.c
> > > > > > b/lib/librte_eal/common/eal_common_timer.c
> > > > > > index dcf26bfea..1358bbed0 100644
> > > > > > --- a/lib/librte_eal/common/eal_common_timer.c
> > > > > > +++ b/lib/librte_eal/common/eal_common_timer.c
> > > > > > @@ -69,7 +69,7 @@ estimate_tsc_freq(void)
> > > > > > 	/* assume that the sleep(1) will sleep for 1 second */
> > > > > > 	uint64_t start = rte_rdtsc();
> > > > > > 	sleep(1);
> > > > > > -	return rte_rdtsc() - start;
> > > > > > +	return RTE_ALIGN_MUL_NEAR(rte_rdtsc() - start, 1E7);
> > > 
> > > The 1E7 is a magic number convert this to a meaningful define.
> > 
> > 1E7 ~ 10Mhz will convert to a macro.
> > 
> > > > > > }
> > > > > > 
> > > > > > void
> > > > > > @@ -83,7 +83,7 @@ set_tsc_freq(void)
> > > > > > 	if (!freq)
> > > > > > 		freq = estimate_tsc_freq();
> > > > > > 
> > > > > > -	RTE_LOG(DEBUG, EAL, "TSC frequency is ~%" PRIu64 "
> > > > > > KHz\n", freq
> > > > > > / 1000);
> > > > > > +	RTE_LOG(INFO, EAL, "TSC frequency is ~%" PRIu64 "
> > > > > > Hz\n", freq);
> > > > > > 	eal_tsc_resolution_hz = freq;
> > > > 
> > > > I missed this log will remove it in the next version.
> > > > 
> > > > > > }
> > > > > > 
> > > > > > diff --git a/lib/librte_eal/linuxapp/eal/eal_timer.c
> > > > > > b/lib/librte_eal/linuxapp/eal/eal_timer.c
> > > > > > index bc8f05199..864d6ef29 100644
> > > > > > --- a/lib/librte_eal/linuxapp/eal/eal_timer.c
> > > > > > +++ b/lib/librte_eal/linuxapp/eal/eal_timer.c
> > > > > > @@ -248,7 +248,7 @@ get_tsc_freq(void)
> > > > > > 
> > > > > > 		double secs = (double)ns/NS_PER_SEC;
> > > > > > 		tsc_hz = (uint64_t)((end - start)/secs);
> > > > > > -		return tsc_hz;
> > > > > > +		return RTE_ALIGN_MUL_NEAR(tsc_hz, 1E7);
> > > > > 
> > > > > Maybe I missed an email about this, but why would I want the
> > > > > TSC
> > > > > hz
> > > > > rounded here? I do not mind the macro just the fact that we
> > > > > are
> > > > > changing TSC hz value. If the TSC value is wrong then we need
> > > > > to
> > > > > fix
> > > > > the value, but I do not see it being wrong here.
> > > > 
> > > > Since in this function nanosleep might not be cycle accurate we
> > > > need to
> > > > round it up.
> > > > 
> > > > Please note that estimation only applies
> > > > when  get_tsc_freq_arch()
> > > > fails. i.e there is no CPU instruction that specifies the
> > > > cyc/sec.
> > > > 
> > > > As I mentioned in the patch notes
> > > > "Useful in case of ARM64 if we enable
> > > > RTE_ARM_EAL_RDTSC_USE_PMU,
> > > > get_tsc_freq_arch() will return 0 as there is no instruction to
> > > > determine the clock of PMU and eal falls back to
> > > > sleep(1)/nanosleep.” 
> > > 
> > > OK, I looked at the changes and the code for setting the TSC
> > > again. I
> > > would have not handled the setting of TSC in the way it was done,
> > > but
> > > that is not your problem. I agree the changes do look ok, the
> > > only
> > > problem I have is the new macro will roundup or down depending on
> > > the
> > > value. In your statement you are wanting to roundup the values.
> > > 
> > > If the sleep/nanosleep is less than a second for some reason,
> > > then
> > > your macro would round it down is that what we wanted? I guess my
> > > point is you are assuming the TSC calculation will always be less
> > > than a second (with sleep) and the macro would round up depending
> > > on
> > > the value calculated using the sleep/nanosleep.
> > > 
> > > I was playing with these MUL macros and I am not sure they do
> > > what we
> > > expect in the case of the multiple value is much closer to the
> > > value
> > > passed.
> > > 
> > > If we have a v = 10001 and multiple to 1000 we have the
> > > following:
> > > 
> > > RTE_ALIGN_MUL_CEIL(10001, 1000)
> > > 	(10001 + (1000 - 1)) / (1000 * 1000)
> > ((10001 + (1000 - 1)) / 1000) * 1000
> > > 	(10001 + 999)        / 1000000
> > > 	20000                / 1000000
> > > Result: 0
> > 
> > ((10001 + (1000 - 1) / 1000) * 1000
> > ((10001 + 999) / 1000) * 1000
> > (11000/1000) * 1000
> > 11 * 1000 
> > 
> > Result : 11000
> > 
> > > RTE_ALIGN_MUL_FLOOR(10001, 1000)
> > > 	(10001 / (1000 * 1000))
> > (10001 / 1000) * 1000
> > > 	(10001 / 1000000)
> > > Result: 0
> > 10.001 * 1000
> > 
> > Result : 10000
> 
> Ooops, too many parans and missed it.
> 
> Then we can get a new version and that should be OK.

Yup, thanks for reviewing :-).

> 
> I will add my $0.02 then:
> 
> Reviewed-by: Keith Wiles<keith.wiles>
> 
> > > Unless I am wrong here the value v must be over a 1,000,000 to
> > > make
> > > these macros work or the value v to be greater than (mul * mul)
> > > in
> > > all cases or zero is the result. It may work for the TSC values
> > > as we
> > > are using a small mul value compared to the TSC value. If DPDK
> > > was
> > > ported to a slower machine it could be also zero.
> > 
> > Unless we have machines that run at freq < 10Mhz this scheme will
> > always work.
> > If we have such machines lets hope that they have a CPU instruction
> > that tells us the cyc/sec.
> > 
> > > I think we need to fix the macros and rethink how TSC is set
> > > here.
> > > 
> > > > > > 	}
> > > > > > #endif
> > > > > > 	return 0;
> > > > > > --
> > > > > > 2.21.0
> > > > > > 
> > > > > 
> > > > > Regards,
> > > > > Keith
> > > 
> > > Regards,
> > > Keith
> > > 
> > 
> > Regards,
> > Pavan.
> 
> Regards,
> Keith
>